Reactive HiPIMS deposition of Al-oxide thin films using W-alloyed Al targets
S. Kagerer, L. Zauner, Tomasz Wójcik, S. Kolozsvári, Tomáš Kozák, Jiří Čapek, P. Zeman, H. Riedl, P.H. Mayrhofer
Abstract
The outstanding oxidation resistance, thermo-mechanical stability, and chemical inertness of alumina, but also the synthesis of phase pure polymorphs attract particular attention in academia and industry. Especially, the difficulties regarding the synthesis of α- or γ-structured Al2O3 by physical vapor deposition techniques are still strong limitations. Within this study, we investigated in detail the influence of 2 at.% tungsten in the Al-target on the process stability and phase formation during reactive DC magnetron sputtering as well as high power impulse magnetron sputtering (HiPIMS) of Al2O3-based coatings. The small addition of W to the Al target allows to increase the oxygen partial pressure by more than 200% while maintaining a stable deposition process. Ion mass spectroscopy measurements yield a promising high fraction of 16O+ and 32O2+, when operating the W-containing target in the metal-to-poisoned transition mode. A significant increase of 16O+ is further provided by the target surface oxide in poisoned mode. Detailed time-of-flight ion mass spectroscopy investigations during one HiPIMS pulse show a clear temporal separation of the individual ions arriving at the substrate plane during the pulse on-time, allowing for controlled ion attraction by synchronizing the bias pulse to the discharge impulse. Equal amounts of 27Al+ and 32O2+ can be attracted using a bias on-time between 400 μs and 900 μs in the “off-time” (after glow) leading to a dense and nano-crystalline coating. Detailed electron microscopy investigations show the presence of metallic phase fractions for higher duty cycles (7.5%). Decreasing the duty cycle to 3.75% leads to amorphous coatings when operating the Al-target at the highest oxygen partial pressure in metallic mode.